05000416/LER-2012-005
| Grand Gulf Nuclear Station | |
| Event date: | 06-13-2012 |
|---|---|
| Report date: | 08-13-2012 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 4162012005R00 - NRC Website | |
A. REPORTABLE OCCURRENCE
This LER is being submitted pursuant to 50.73(a)(2)(i)(B) for a condition prohibited by Technical Specification (TS) Limiting Condition of Operation (LCO) 3.3.1.1, Reactor Protection System (RPS) Instrumentation for the Inoperable Average Power Range Monitor (APRM) [IGj Function. As the LCO was not entered, the condition existed for a time period beyond TS allowances and 50.73(a)(2)(v) for an Event or Condition That Could Have Prevented Fulfillment of a Safety Function.
B. INITIAL CONDITIONS
On June 13, 2012 the date of event discovery, the reactor was in Mode 1 operating at approximately 12 — 15 percent (%) Rated Thermal Power (RTP). There were no additional inoperable structures, systems, or components at the start of the event that contributed to this event. The reportable event occurred from the time the reactor was placed into Mode 2 on June 6, 2012 until the reactor was placed into Mode 1 on June 13, 2012.
C. DESCRIPTION OF OCCURRENCE
The APRM channels receive input signals from the Local Power Range Monitors (LPRMs) [IGj channels and provide a continuous indication of average reactor power from a few percent to greater than rated reactor power. The APRM channels average these LRPM signals to provide a continuous indication of average reactor power from a few percent to greater than RTP. In Mode 2, the intermediate range monitors (IRM) and the APRM channels provide separate trip signals to the reactor protection system (RPS) for reactor power transients. Additionally, reactor vessel high pressure is an independent variable and for this condition provides diverse trip initiating circuits for the protective action (scram).
During startup from Refueling Outage 18 (RF18) the APRMs were set to indicate flux lower than the actual power level resulting in the system being inoperable during Mode 2 due to the APRM setdown high flux scram setpoint being outside of TS limits.
In RF18 Grand Gulf Nuclear Station (GGNS) upgraded the Power Range Neutron Monitoring (PRNM) [IGj system to a new, digital, General Electric-Hitachi (GEH) designed Nuclear Measurement Analysis & Control (NUMAC) system as part of the Extended Power Uprate (EPU) station upgrades. Included in this upgrade was the installation of new equipment, initial setup, and document/procedure revisions.
After entering Mode 1 during startup from RF18 it was noted that the APRM flux indication did not match what the heat balance was indicating. While the APRMs were indicating approximately 6.5% flux, other indications showed that Core Thermal Power (CTP) was actually at approximately 12 —15% RTP. PRNM receives inputs from the LPRMs installed in the core and it applies a gain to the signals to indicate the local power. The gain applied at this point is referred to as the LRPM gain. These signals are then averaged and another gain is applied to have the output collate with CTP. This gain is referred to as the APRM gain.
After noting the discrepancy, actions were taken to validate the heat balance inputs. After the heat balance inputs were confirmed as accurate, the APRM gains were adjusted such that indicated flux matched the measured heat balance. APRM gains are adjusted to within +/- 2% of the heat balance per TS Surveillance Requirement 3.3.1.1.2.
Because the APRM gains were set such that APRM flux was non-conservatively measured throughout Mode 2, the setdown high flux scram set at 15% flux would not have caused a scram until after exceeding the TS allowable limit.
D. APPARENT CAUSE
The apparent cause of this condition was the differing operating characteristics between the old system and the new system that were not noted during the Engineering Change (EC) process, therefore they were not adequately addressed by procedure revisions/work order instructions. This is a knowledge based task, and thus considered a human performance error. The personnel responsible for the procedure revision and the associated technical review were working for the EPU project as contractors and are no longer on site.
Additionally, there were two contributing causes to this event:
1. Multiple LPRMs were failed downscale and were not bypassed.
2. LPRM gains for new detectors were set slightly lower than the value recommended by GEH.
E. CORRECTIVE ACTIONS
Immediate Corrective Actions:
1. Any LPRMs reading downscale were bypassed (06/17/2012).
2. Perform a Traversing In-core Probe (TIP) set (06/21/2012).
3. Applied a generic Gain Adjustment Factor (GAF) to all the LRPMs. This increased each LRPM flux while keeping them proportional to one another. This allowed APRM gains to then be adjusted to match CTP (06/18/2012).
Corrective Actions to Prevent Recurrence:
1. Revise procedure to set the APRM gains to the maximum level (3.000) prior to entering Mode 2 (08/31/2012).
2. Revise procedure to incorporate initial LRPM gain setting for detector replacements (08/31/2012).
F. SAFETY ASSESSMENT
During Mode 2 the intermediate range monitors (IRMs) are required to be operable. One of the design functions of the IRMs is to generate a reactor trip signal on high neutron flux to prevent fuel damage resulting from anticipated or abnormal operational transients that may occur while operating in the intermediate (heating) range of reactor power. Therefore, during Mode 2 we are required to have two neutron monitoring systems capable of causing a reactor trip, both of which are designed to prevent fuel damage due to transient conditions. Although the APRMs were not properly configured to cause a scram prior to the Technical Specification allowable value, the IRMs were capable of causing a reactor trip at their calibrated values throughout Mode 2. The PRNM system was capable of sending a scram signal to the reactor protection system at all times. Additionally, reactor vessel high pressure is an independent variable and for this condition provides diverse trip initiating circuits for the protective action (scram).
During startup in Mode 2, the IRM and the high reactor pressure trip functions were operable. Therefore reactor power transients would have been mitigated by these functions. The APRM Neutron Flux High (Setdown) function is not directly credited in any safety analyses this event did not adversely affect plant safety or the health and safety of the public.
G. ADDITIONAL INFORMATION
Previous Occurrences — A search of the Paperless Condition Reporting System (PCRS) for the last five years revealed no prior or similar events were the APRMs were set to indicate flux lower than actual power.
No License Event Reports (LERs) have been submitted for a similar event in the last five years.